A cylindrical item having an internally flanged diameter designed to retain a lens or window in the objective end, or a diopter scale at the viewing end of an optical instrument. It may have one or more of the following characteristics: internal thread or machined for a press fit, method of securing, and/or knurled periphery. For externally threaded items, see RETAINER, OPTICAL ELEMENT. Excludes CELL, OPTICAL ELEMENT; NUT, PLAIN KNURLED; NUT, PLAIN ROUND; and RING, RETAINING.